diff --git a/base_interface.go b/base_interface.go
index 6fdaf3cf..8d64756f 100644
--- a/base_interface.go
+++ b/base_interface.go
@@ -29,7 +29,8 @@ type options struct {
 
 // Client is the Twilio SendGrid Go client
 type Client struct {
-	rest.Request
+	apiKey       string
+	emailOptions TwilioEmailOptions
 }
 
 func (o *options) baseURL() string {
@@ -57,19 +58,41 @@ func requestNew(options options) rest.Request {
 
 // Send sends an email through Twilio SendGrid
 func (cl *Client) Send(email *mail.SGMailV3) (*rest.Response, error) {
-	return cl.SendWithContext(context.Background(), email)
+	return cl.SendWithContext(context.Background(), email, nil)
 }
 
-// SendWithContext sends an email through Twilio SendGrid with context.Context.
-func (cl *Client) SendWithContext(ctx context.Context, email *mail.SGMailV3) (*rest.Response, error) {
-	cl.Body = mail.GetRequestBody(email)
+// SendWithHeaders sends an email through Twilio SendGrid with additional headers
+func (cl *Client) SendWithHeaders(email *mail.SGMailV3, headers map[string]string) (*rest.Response, error) {
+	return cl.SendWithContext(context.Background(), email, headers)
+}
+
+// SendWithContext sends an email through Twilio SendGrid with context.Context
+func (cl *Client) SendWithContext(ctx context.Context, email *mail.SGMailV3, headers map[string]string) (*rest.Response, error) {
+	var request rest.Request
+
+	if cl.apiKey != "" {
+		request = GetRequest(cl.apiKey, "/v3/mail/send", "")
+	} else if cl.emailOptions != (TwilioEmailOptions{}) {
+		request = GetTwilioEmailRequest(cl.emailOptions)
+	} else {
+		return nil, errors.New("no API key or email options provided")
+	}
+
+	request.Method = "POST"
+
+	// Add any custom headers provided by the caller.
+	for k, v := range headers {
+		request.Headers[k] = v
+	}
+
+	request.Body = mail.GetRequestBody(email)
 	// when Content-Encoding header is set to "gzip"
 	// mail body is compressed using gzip according to
 	// https://docs.sendgrid.com/api-reference/mail-send/mail-send#mail-body-compression
-	if cl.Headers["Content-Encoding"] == "gzip" {
+	if request.Headers["Content-Encoding"] == "gzip" {
 		var gzipped bytes.Buffer
 		gz := gzip.NewWriter(&gzipped)
-		if _, err := gz.Write(cl.Body); err != nil {
+		if _, err := gz.Write(request.Body); err != nil {
 			return nil, err
 		}
 		if err := gz.Flush(); err != nil {
@@ -79,9 +102,9 @@ func (cl *Client) SendWithContext(ctx context.Context, email *mail.SGMailV3) (*r
 			return nil, err
 		}
 
-		cl.Body = gzipped.Bytes()
+		request.Body = gzipped.Bytes()
 	}
-	return MakeRequestWithContext(ctx, cl.Request)
+	return MakeRequestWithContext(ctx, request)
 }
 
 // DefaultClient is used if no custom HTTP client is defined
diff --git a/go.coverage.sh b/go.coverage.sh
index fadb3cc8..07442729 100755
--- a/go.coverage.sh
+++ b/go.coverage.sh
@@ -4,7 +4,7 @@ set -e
 echo > coverage.txt
 
 for d in $(go list ./... | grep -v -E '/vendor|/examples|/docker'); do
-    go test -coverprofile=profile.out -covermode=atomic "$d"
+    go test -race -coverprofile=profile.out -covermode=atomic "$d"
     if [ -f profile.out ]; then
         cat profile.out >> coverage.txt
         rm profile.out
diff --git a/sendgrid.go b/sendgrid.go
index 1c50451c..01d242d2 100644
--- a/sendgrid.go
+++ b/sendgrid.go
@@ -2,8 +2,9 @@ package sendgrid
 
 import (
 	"errors"
-	"github.com/sendgrid/rest"
 	"net/url"
+
+	"github.com/sendgrid/rest"
 )
 
 // sendGridOptions for CreateRequest
@@ -51,9 +52,7 @@ func createSendGridRequest(sgOptions sendGridOptions) rest.Request {
 
 // NewSendClient constructs a new Twilio SendGrid client given an API key
 func NewSendClient(key string) *Client {
-	request := GetRequest(key, "/v3/mail/send", "")
-	request.Method = "POST"
-	return &Client{request}
+	return &Client{apiKey: key}
 }
 
 // extractEndpoint extracts the endpoint from a baseURL
diff --git a/sendgrid_test.go b/sendgrid_test.go
index 57b52709..086eb1e3 100644
--- a/sendgrid_test.go
+++ b/sendgrid_test.go
@@ -9,6 +9,7 @@ import (
 	"os"
 	"strconv"
 	"strings"
+	"sync"
 	"testing"
 	"time"
 
@@ -1640,10 +1641,52 @@ func Test_test_mail_batch__batch_id__get(t *testing.T) {
 	assert.Equal(t, 200, response.StatusCode, "Wrong status code returned")
 }
 
+func Test_test_client_send_is_thread_safe(t *testing.T) {
+	apiKey := "SENDGIRD_APIKEY"
+	const numRequests = 5
+	var wg sync.WaitGroup
+	client := NewSendClient(apiKey)
+
+	// Launch multiple goroutines to send concurrent requests
+	for i := 0; i < numRequests; i++ {
+		wg.Add(1)
+		go func(i int) {
+			defer wg.Done()
+			from := &mail.Email{
+				Name:    "Name",
+				Address: "sam.smith@example.com",
+			}
+
+			to := &mail.Email{
+				Name:    "Recipient",
+				Address: "jane.doe@example.com",
+			}
+
+			email := &mail.SGMailV3{
+				From: from,
+				Personalizations: []*mail.Personalization{
+					{
+						To:      []*mail.Email{to},
+						Subject: "Subject",
+					},
+				},
+				Content: []*mail.Content{
+					{
+						Type:  "text/plain",
+						Value: "Value",
+					},
+				},
+			}
+
+			client.Send(email)
+		}(i)
+	}
+	wg.Wait()
+}
+
 func Test_test_send_client_with_mail_body_compression_enabled(t *testing.T) {
 	apiKey := "SENDGRID_API_KEY"
 	client := NewSendClient(apiKey)
-	client.Headers["Content-Encoding"] = "gzip"
 
 	emailBytes := []byte(` {
 		"asm": {
@@ -1780,8 +1823,10 @@ func Test_test_send_client_with_mail_body_compression_enabled(t *testing.T) {
 	email := &mail.SGMailV3{}
 	err := json.Unmarshal(emailBytes, email)
 	assert.Nil(t, err, fmt.Sprintf("Unmarshal error: %v", err))
-	client.Request.Headers["X-Mock"] = "202"
-	response, err := client.Send(email)
+
+	headers := map[string]string{"Content-Encoding": "gzip", "X-Mock": "202"}
+
+	response, err := client.SendWithHeaders(email, headers)
 	if err != nil {
 		t.Log(err)
 	}
@@ -1929,8 +1974,7 @@ func Test_test_send_client(t *testing.T) {
 	email := &mail.SGMailV3{}
 	err := json.Unmarshal(emailBytes, email)
 	assert.Nil(t, err, fmt.Sprintf("Unmarshal error: %v", err))
-	client.Request.Headers["X-Mock"] = "202"
-	response, err := client.Send(email)
+	response, err := client.SendWithHeaders(email, map[string]string{"X-Mock": "202"})
 	if err != nil {
 		t.Log(err)
 	}
diff --git a/twilio_email.go b/twilio_email.go
index 52981a41..1d39c848 100644
--- a/twilio_email.go
+++ b/twilio_email.go
@@ -16,9 +16,8 @@ type TwilioEmailOptions struct {
 
 // NewTwilioEmailSendClient constructs a new Twilio Email client given a username and password
 func NewTwilioEmailSendClient(username, password string) *Client {
-	request := GetTwilioEmailRequest(TwilioEmailOptions{Username: username, Password: password, Endpoint: "/v3/mail/send"})
-	request.Method = "POST"
-	return &Client{request}
+	emailOptions := &TwilioEmailOptions{Username: username, Password: password, Endpoint: "/v3/mail/send"}
+	return &Client{emailOptions: *emailOptions}
 }
 
 // GetTwilioEmailRequest create Request
diff --git a/twilio_email_test.go b/twilio_email_test.go
index 51f392c6..16d14cf7 100644
--- a/twilio_email_test.go
+++ b/twilio_email_test.go
@@ -13,8 +13,9 @@ import (
 
 func TestNewTwilioEmailSendClient(t *testing.T) {
 	mailClient := NewTwilioEmailSendClient("username", "password")
-	assert.Equal(t, "https://email.twilio.com/v3/mail/send", mailClient.BaseURL)
-	assert.Equal(t, "Basic dXNlcm5hbWU6cGFzc3dvcmQ=", mailClient.Headers["Authorization"])
+	request := GetTwilioEmailRequest(mailClient.emailOptions)
+	assert.Equal(t, "https://email.twilio.com/v3/mail/send", request.BaseURL)
+	assert.Equal(t, "Basic dXNlcm5hbWU6cGFzc3dvcmQ=", request.Headers["Authorization"])
 }
 
 func TestGetTwilioEmailRequest(t *testing.T) {